Fire-resistant landscaping: a firescaping guide

Firescaping starts at the house and works out

Order of work, per CSU's Fire-Adapted Landscaping Practices: harden the structure first (roofing, siding, decks and fences attached to the house), then Zone 1, then outward. The reason is embers. CSU says structures usually ignite from firebrands, the embers thrown off by burning vegetation and buildings, and those can travel long distances.

The three zones are measured from the edge of the house, eaves, stairs and decks included:

• Zone 1, 0-5 ft. Ideally nothing that burns.
• Zone 2, 5-30 ft. Small islands of green, ignition-resistant plants with breaks between them.
• Zone 3, 30-100 ft. Thinned and spaced so a fire stays on the ground.

Together they are what CSU calls the home ignition zone. Our defensible space guide has the zone diagram and the full rules for each ring.

Fire resistant mulch: what to use and what to skip

Mulch is where good intentions go wrong, because the most common mulches, wood chips and bark, are the ones CSU says carry fire. What CSU's Fire-Adapted Landscaping Practices says, by zone:

• Zone 1 (0-5 ft). No wood chips, bark, pine needles or shredded rubber. The ideal surface is non-combustible: a concrete sidewalk, flagstone or pavers, brick, gravel or bare ground. Gravel has one catch: it is harder to rake fine debris out of gravel than off a smooth surface, so it needs regular cleaning.
• Zone 2 (5-30 ft). Mulch is fine here for moisture and weed control, and CSU prefers gravel, because wood chips, bark, pine needles and shredded rubber readily carry fire.
• Under any mulch. No landscape fabric, for plant health and for fire resistance.
• At fences. Clear wood mulch, needles, leaf litter and small twigs from the base of all fences.

So the fire resistant mulch, in CSU's terms, is gravel or other rock. The fact sheet does not name or rate any treated or branded wood mulch, so we do not either. It also warns against swinging too far: an all-rock landscape is not necessarily the most fire-resistant, since green plants store water and slow the spread of fire. Rock and gravel are for the first 5 feet and for breaks between plantings, not a reason to pave the yard.

Spacing and ladder fuels

Fire needs a path. CSU's term for the path is fuel continuity: how burnable things sit relative to each other. Spacing breaks it up.

• Zone 2 plantings. Small, irregular clusters or islands, not large masses, with decorative rock, gravel and stepping-stone paths between them.
• Shrubs. Space them at least 2 1/2 times their mature height, measured from the outer branches. For a shrub that grows 6 feet tall, that is 15 feet or more.
• Ladder fuels. CSU says to remove shrubs beneath trees, because they can act as ladder fuels that carry a fire from the ground into the treetops. Keep perennials and grasses short under trees too.
• Evergreens. Do not plant new evergreens in Zone 2. Existing ones should be thinned to at least 10 to 15 feet between canopies.
• Limbing up. Prune so the lowest limbs are at least 6 feet off the ground, but never remove more than a third of a tree's total height. That caution matters most for small trees like piñon and juniper.
• Zone 3 trees. Plant new ones on a 20 to 25 foot spacing to leave room for growth. On steep ground, leave even more room between crowns.

Watering and upkeep through the season

A fire-resistant landscape is not a one-time install. CSU puts it plainly: even plants considered ignition resistant can lose that quality over time, so the landscape has to be maintained to keep it. CSU Extension's Fact Sheet 6.305 adds that moisture content is the single most important factor in how readily a plant ignites. The maintenance list in CSU's Fire-Adapted Landscaping Practices:

• Water. Keep plants at their optimal moisture. If water rights permit, CSU suggests deeply watering trees and shrubs every 20 to 30 days during the growing season. If you cannot irrigate, choose ignition-resistant plants that survive and stay green without supplemental water.
• Dead material. Trim diseased and dead branches and plants, and do not let dry plant debris pile up. Cut back annuals and perennials after they go to seed or when the stems dry out.
• Litter. Clean up leaves and other litter through the season. Clear gutters and roofs after every significant storm and at least each spring and fall. Check Zones 1 and 2 weekly.
• Lawn. Mow to 4 inches or less in Zone 2.
• Cured grass. Dry grass is fine fuel that ignites easily and carries fire fast, native or not. On the Front Range the grassland fire risk runs September to April, when grass is dry and winds are common, and CSU says the most important cut is in the fall at the start of that window. Zone 3 grass can stay unmowed if you like, with that caveat.

Existing plants: keep, thin or remove

You probably already have plants on the list of troublemakers, and CSU does not ask you to pull everything. CSU Extension's Fact Sheet 6.305 says most of Colorado's native vegetation is adapted to fire and is ignitable, and that common ignitable plants score below 8. Its examples: junipers, pines, firs and spruces among trees, and Gambel oak, three-leaf sumac and mountain mahogany among shrubs. Conifers tend to be ignitable because of their oil and pitch, even when well watered.

A practical way to sort what you have:

• Within 5 feet of the house: branches should stay at least 5 feet from the structure. CSU's ideal for this strip is non-combustible ground, and if you keep plants there, CSU Extension's Fact Sheet 6.305 says only those scoring 8 or higher.
• Evergreens in Zone 2: CSU's rule is no new ones. Existing ones should be in very good condition, with little to no deadwood, and thinned to 10 to 15 feet between canopies.
• Dead or dying trees and shrubs: remove them. That one is unambiguous.
• Everything else: prune, thin, limb up and clear the litter beneath, which is the maintenance list above.

The score-8 bar is CSU's guidance for what you plant new in the first 30 feet; for plants already there, its answer is management. A plant that CSU does not score has no verdict either way, so no score does not mean it burns. When in doubt, your county Extension office or Colorado State Forest Service field office can look at a specific property.

Plant picks by zone

Zones 1 and 2 (0-5 ft and 5-30 ft) share one plant bar: a CSU score of 8 or higher. 49 of the 53 plants on our Colorado list meet it. The other 4 are on Plant Select's fire-resistant list without a CSU score, so we point them at Zone 2 only.

• All 53 picks on the Colorado page, with save buttons.
• All 15 fire-resistant groundcovers, ranked, with a side-by-side table.
• Filter by type in the finder, with the fire zone preset: groundcovers, perennials.

For trees, CSU favors healthy deciduous ones in Zones 2 and 3, and we do not have a ranked tree list because the CSU table does not cover them. The reasoning is on the Colorado page.